Ni0.02Zn0.98O Nanocomposite with High Solar Light

Sep 13, 2014 - Photocatalytic removal of toxic textile dyes from wastewater is a challenge because of the relatively low efficiency of photocatalysts...
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Polyacrylamide/Ni0.02Zn0.98O Nanocomposite with High Solar Light Photocatalytic Activity and Efficient Adsorption Capacity for Toxic Dye Removal Amit Kumar,*,† Gaurav Sharma,†,‡ Mu Naushad,§ Pardeep Singh,† and Susheel Kalia∥ †

School of Chemistry, Shoolini University, Solan, Himachal Pradesh-173212 India College of Forestry, Dr. Y. S. Parmar University of Horticulture and Forestry, Nauni, Solan, Himachal Pradesh-173230, India § Department of Chemistry, College of Science, Building #5, King Saud University, Riyadh, Saudi Arabia ∥ Department of Chemistry, Bahra University, Shimla Hills, Waknaghat, Solan, Himachal Pradesh-173215, India ‡

S Supporting Information *

ABSTRACT: Photocatalytic removal of toxic textile dyes from wastewater is a challenge because of the relatively low efficiency of photocatalysts. There has been sustained interest in a variety of cheap, hybrid, and efficient nanomaterials for wastewater treatment. In the present work, a novel photocatalyst polyacrylamide/Ni0.02Zn0.98O (PAM/NZP) was synthesized successfully by addition of nanoparticles during polymerization of acrylamide in aqueous medium using ammonium persulfate and N,N′methylenebis(acrylamide). The material possesses excellent photoactivity and high adsorption capacity.The present investigation describes the applicability of PAM/NZP for removal of malachite green (MG) and rhodamine B (RB) from aqueous solution. The effect of adsorption capacity of cross-linked polyacrylamide on photocatalytic activity of Ni0.02Zn0.98O was also studied. The materials were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, high-resolution transmission electron microscopy, small area electron diffraction, thermal gravimetric analysis, and ultraviolet−visible spectroscopy. The optical band gap was 3.17 eV for NZP and 3.07 eV for PAM/NZP, which is quite lower than that of bare ZnO. The simultaneous adsorption and photocatalysis proved to be a better reaction condition for photodegradation of both the dyes in the presence of PAM/NZP under natural sunlight irradiation. A significant removal efficiency of 99.17% for RB and 96.55% for MG was achieved in 2 h of solar illumination in the presence of the nanocomposite. In addition, the nanocomposite has a high recycling efficiency.

1. INTRODUCTION Dye-contaminated wastewater from the textile industry contributes significantly to various diseases, such as hypertension, hemolysis, organ damage, respiratory disorders, etc., in humans and aquatic animals and plants. However, various contaminants such as natural organic matters, industrial dyes, microorganisms and heavy metals further worsen this problem. Various textile industries, pulp mills, and dye manufacturing facilities release highly colored wastewaters which have aroused environmental concerns all over the planet. The immense development of the textile industry has contributed to the significant amount of water pollution, which is mainly due to the disposal of industrial wastes, such as various inorganic and organic dyes, into water resources. Dyes are broadly classified as anionic (acidic), cationic (basic), or nonionic.1 The economic removal of color from textile industry effluents remains a major problem. Many dyes although not toxic, present an aesthetic problem and reduce photosynthetic activity in the water into which they are discharged. Most of the dyes are toxic or carcinogenic and can cause allergic dermatitis, skin irritation, mutation, etc.2−4 Some dyes are designed for their chemical stability and do not readily undergo bioch emical d egrad ation. Rhodamine B (C28H31ClN2O3, C.I. no. 45170:1) is one of the most important dyes of the xanthene group, which is highly water-soluble. It causes irritation to the skin, eyes, and respiratory tract and is © 2014 American Chemical Society

widely used in many industrial processes, such as paper dyeing and the production of dye lasers.5,6 Malachite green (C23H25N2Cl, C.I. no. 42000) is a green crystal powder with luster and is highly soluble in water.4 It acts as a respiratory enzyme poison in fish. It decreases food intake, causes damage to the liver and kidneys, and causes infections in skin, eyes, and bones. There is thus great interest in affordable technologies capable of removing these toxic dyes and pathogens from water. Various experiments have demonstrated the efficacy and benefits of various techniques such as adsorption, 7−9 ozonation,10,11 coagulation,12 electrochemical oxidation,13 and photocatalysis14−16 for removal of dyes. Many of these methods suffer drawbacks of simply transferring the pollutant from one phase to another (e.g., adsorption), large energy requirements (e.g., thermal destruction), and requiring long treatment periods (e.g., biological treatment). Adsorption processes have been most exploited so far because of the merits of ease, efficiency, and economy. However, to combat persistent pollutants in bodies of water, various other methods have been used as an alternative to adsorption.17 Recently, advanced Received: Revised: Accepted: Published: 15549

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synthesized an inexpensive nanomaterial based on Ni-doped ZnO and cross-linked polyacrylamide which has dual capabilities of adsorptional photocatalysis and antimicrobial action. The work deals with exploitation of Ni0.02Zn0.09O and its nanocomposite with polyacrylamide (PAM/NZP) for removal of rhodamine B and malachite green by coupled adsorption and photocatalysis.

oxidation processes (AOPs) have been widely demonstrated to be reliable for wastewater treatment because they have a high capacity to oxidize nearly all organic pollutants.18,19 Increasing regulations worldwide for treatment of wastewater have forced scientists to find more innovative and eco-friendly techniques for removal of toxic dyes.20 Various nanomaterials have been found to be excellent photocatalysts because of semiconducting nature, large surface area, and tunable optical properties. Among many semiconductors, zinc oxide (ZnO) has been considered to be of great importance because of its ecofriendliness. However, a general limitation of the photocatalytic process is the low quantum efficiencies caused by two critical factors, which are the recombination and the insufficient utilization of solar energy. Considerable efforts have been made to suppress the recombination and hence to enhance the charge carrier separation and the overall efficiency, including modification of the physicochemical properties of the semiconductor materials, such as particle size, surface area, porosity, and crystallinity, and optimization of the experimental conditions during photocatalytic reactions, such as pH values, illumination conditions, and catalyst loading. ZnO has been investigated in recent years also as a special photocatalyst, leading to the total mineralization of a wide range of organic dyes. In some cases, ZnO exhibits even better photocatalytic efficiency than TiO221,22 because of its high efficiency of generation, mobility, and separation of photoinduced electrons and holes. The effect of transition-metal ion dopants on the photocatalytic activity is the dynamics of electron−hole recombination and interfacial charge transfer. The dopant ions can function as both hole and electron traps, or they can mediate interfacial charge transfer.23 The reduction in band gap shifts the absorption to the visible region, thus increasing environmental applicability. Mohammad et al.24 showed that the rate of dye degradation of methylene blue in the case of Mn-doped ZnO nanoparticles was double that of undoped nanoparticles. Nenavathu et al.25 studied the effect of selenium doping on photocatalytic activity of ZnO nanoparticles against tryptan blue dye. However, significant results were obtained at high dopant concentrations. Similarly, higher dopant concentrations of copper as dopant were successful in bringing out decent degradation of dye.26 From various earlier works we conclude that various dopants have been used to enhance the photodegrataion activity of ZnO nanoparticles; however, high and moderate dopant concentrations have been used. Recently, adsorption and photocatalytic oxidation processes have been used as effective and reliable methods for dye removal. The adsorption is advantageous in terms of cost, time consumption, and recovery of adsorbent as well as adsorbate. Although adsorption is a widely exploited method for dye effluent treatment, it only transfers dye from aqueous to solid phase thus leading to secondary pollution. Several polymers with different functional groups have attracted great interest for organic pollutant removal because of their high adsorption capacities, regeneration abilities, and continuous reuse. Crosslinked polyacrylamide (PAM) is a superabsorbent polymer and a common flocculant for wastewater treatment. An advanced oxidation process initiated by photocatalytic degradation can offer a better solution for decolorization, breakdown, and mineralization of dyes. Establishing rational design principles is essential for transforming exciting properties of nanomaterials into costeffective applications. Following these guidelines we have

2. MATERIALS AND METHODS 2.1. Materials. All the chemicals used were of analytical grade. Acrylamide, zinc nitrate, nickel nitrate, ammonium persulfate, and N,N′-methylenebis(acrylamide) were procured from Merck India and used without further purification. Malachite green and rhodamine B were purchased from Sigma-Aldrich. 2.2. Synthesis of Ni0.02Zn0.98O Nanoparticles. For preparing the Ni0.02Zn0.98O nanoparticles (NZP), zinc nitrate and nickel nitrate were mixed in stoichiometric ratio and dissolved in 100 mL of double-distilled water. Citric acid (4.66 g) was added as a gelling agent. The pH was maintained at 7 by adding NH3 solution dropwise. The mixture was allowed to stir for 3 h. When a homogeneous gel started to form, the mixture was dried at 80 °C to form a gel. The powder thus obtained was grinded and sintered at 600 °C for 2 h. 2.3. Synthesis of PAM/NZP Nanocomposite. The PAM/NZP nanocomposite was prepared by addition of synthesized NZP nanoparticles during free radical-initiated polymerization of acrylamide. For this, a fine suspension of 500 mg of Ni0.02Zn0.98O nanoparticles was prepared in 50 mL of distilled water. Then, 30 mL of 0.5 M acrylamide solution, 5% ammonium persulfate (APS), and 7% N,N′-methylenebis(acrylamide) (cross-linker) solution prepared in distilled water were added dropwise. The reaction mixture was stirred continuously at 65 °C for 2 h. A gel was obtained. It was washed several times with distilled water to remove any homopolymer if formed. The solution was then centrifuged to remove soluble impurities and byproducts. The composite was then dried at 55 °C for 5 h in a vacuum oven. 2.4. Characterization. Fourier transform infrared (FTIR) spectra were recorded using a Nicolet 5700 FTIR spectrometer by the KBr pellet method. The phase purity and crystallite size of Ni0.02Zn0.98O and PAM/NZP nanocomposite were determined by an XPERT-PRO X-ray diffractometer using Cu Kα radiation. The surface morphology of samples was studied using a scanning electron microscopy (SEM) QUANTA250 FEI D9393 instrument. High-resolution transmission electron microscopy (HRTEM) and small area electron diffraction (SAED) were performed using an FEI Tecnai F20 transmission electron microscope. Elemental analysis was done by an energy dispersive X-ray (EDX) spectroscopy instrument equipped with SEM. The ultraviolet−visible spectra (UV−vis) were recorded using a Systronics 2202 double beam spectrophotometer. The X-ray diffraction (XRD) studies were used to find the lattice parameters and crystal structure. Scherrer’s formula was used for determination of crystallite size P=

Kλ β cos θ

(1)

where P is the crystallite size, and β the full width at halfmaximum; K = 0.9 and λ = 1.54 Å for Cu Kα radiation The d-spacings are calculated using Bragg’s diffraction law nλ = 2d sin θ 15550

(2)

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2.5. Optical Band Gap Studies. For computation of optical band gap, a suspension of 5 mg of Ni0.02Zn0.98O nanoparticles in ethanol was prepared. The suspension was then ultrasonicated for 1 h. The UV−vis spectrum was then recorded using a double-beam spectrophotometer. The same process was repeated for PAM and PAM/NZP. The band gaps were calculated using the Tauc relation. 2.6. BET Studies. The surface area of nanoparticles and nanocomposites was calculated according to the Brunauer− Emmett−Teller (BET) model NOVA 2200e Quantachrome over a relative pressure range of 0.05−0.90 using nitrogen as a purge gas. 2.7. Dye Removal Test. The photocatalytic activity and effect of adsoprption onto photocatalysis of synthesized nanomaterials were studied using rhodamine B and malachite green as target aqueous pollutants. The dye removal efficiency of samples was analyzed in the dark as well as sunlight in a slurry type batch reactor27 represented in Figure 1. In the

3. RESULTS AND DISCUSSION 3.1. Characterization. 3.1.1. FTIR Analysis. Figure 2 represents the FTIR spectra of PAM, NZP, and PAM/NZP. A strong absorption band at 487.0 cm−1 is observed in the FTIR spectrum of NZP which lies in the Zn−O stretching frequency range.28 A minor peak at 701 cm−1 corresponds to Ni−O stretching.29 In the FTIR spectrum of PAM, the absorption peaks at 3416 and 3190 cm−1, corresponding to O− H stretching and N−H stretching,30 respectively. The characteristic peaks at 1451 cm−1 correspond to −C−N stretching,31 1664 cm−1 to CO stretching,32 and 2930 cm−1 to CH2 stretching. The FTIR spectrum of PAM/NZP indicates that the characteristic peaks of PAM are shifted slightly, which was due to bond formation between ZnO and the polymer. The peaks corresponding to O−H and N−H stretching were shifted to 3410 and 3181 cm−1 in the nanocomposite, respectively. The broad band between 600 and 900 cm−1 refers to N−H wagging bending vibration. This shift in the absorption confirms the bonding of oxygen of metal oxide to NH group in nanocomposite. The peaks at 2927, 2195, 1659, 1452, and 476 cm−1 correspond to −CH2 stretching, CO stretching, C−N stretching, N−H stretching, Zn−O stretching, respectively. This also confirms the bonding of ZnO to the polymer matrix. 3.1.2. XRD Analysis. The XRD patterns of the NZP, PAM, and PAM/NZP are shown in Figure 3. The diffractogram for NZP shows broad peaks at the positions of 31.5431, 34.4401, 36.2844, 46.96271, 56.4532, 62.7247, 66.1932, 67.7198, and 69.1412 which were in good agreement with the standard JCPDS file no. 80-0075 for ZnO.33 The structure can be indexed as the hexagonal wurtzite having space group P63mc.34 No additional peaks corresponding to the secondary phases of nickel oxides were obtained, which indicates that the wurtzite structure is not disturbed by the Ni substitution. The average crystallite size for NZP is 15−22 nm and for PAM/NZP is 25− 32 nm. The XRD pattern of polyacrylamide confirms its amorphous structure. The characteristic peaks of hexagonal structure of NZP are visible in the XRD pattern of the composite, and the composite as a whole is semicrystalline in structure. The Shannon radius of Ni2+ ion (0.055 nm) is only slightly less than that of Zn2+ ion (0.060 nm). Therefore, the substitution of Ni2+ ion for Zn2+ will not result in much distortion in the ZnO lattice. The lattice constants for NZPs as calculated from XRD data are a = b = 3.2447 Å and c = 5.2201 Å. 3.1.3. UV−Visible Analysis. The UV−vis absorption spectra of the NZP, PAM, and PAM/NZP are given in Figure 4a. A broad absorption peak at 387 nm is a characteristic of hexagonal nano zinc oxide. However, as compared to undoped ZnO, a red shift is observed in the case of NZP which is common in transition-metal doped ZnO.35 NZP has a visible absorption higher than that of bare ZnO. The band gaps of nanoparticles and composite were calculated using the Tauc relation36

Figure 1. Schematic diagram of the experimental setup for adsorption and photocatalytic Process.

experiment, the slurry consisting of dye MG (9.27 mg/L) or RB (4.79 mg/L) and catalyst suspension (0.25 mg/mL) was stirred magnetically. The photocatalytic reaction was carried out in a double-walled pyrex glass vessel containing the slurry and surrounded by thermostatic water circulation arrangement to keep temperature in the range of 30 ± 0.5 °C. The pH of the mixture was maintained at 7 using acid or base. In the first experiment, the dye solution containing the photocatalyst was kept in the dark for 1 h to establish adsorption−desorption equilibrium. The system was then exposed to sunlight. After intervals of time, aliquots of 3 mL were taken out and centrifuged to remove the catalyst from the suspension. The absorbance of dye solution was then recorded using a doublebeam spectrophotometer at 620 nm (MG) and 554 nm (RB). The experiments were performed in the month of April with average intensity of sunlight as 29 × 103 ± 100 lx. All experiments were performed in triplicate, and average values were reported. To study the effect of adsorption on photocatalytic activity of the nanomaterials, the coupled effect of adsorption and photocatalysis was also studied on exposing the dye solution containing PAM/NZP to sunlight without keeping it in the dark.

αhν = B(hν − Eg )n

(3)

where α is absorption coefficient (2.303A/l),37 Eg the optical band gap, B the band tailing parameter, and hν the photon energy; n = 1/2 for direct band gap semiconductors. The optical band gap is determined by extrapolating the straight 15551

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Figure 2. FTIR spectra of NZP, PAM, and PAM/NZP.

portion of the curve between (αhν)2 and hν when α = 0. The band gap as calculated from Tauc plots is 3.17 eV for NZP and 3.07 eV for PAM/NZP. The band gap of NZP is lower than that of bare ZnO (3.37 eV), which is due to strong sp−d exchange interactions induced by Ni2+ ions. In addition, dopant ions reduce the recombination chance of excitons. The doping induces the distortion of local electric field, and the photoinduced electrons and holes could be trapped around the dopant. The decrease in optical band gap thus enhances the visible region applications of ZnO nanocrystals. 3.1.4. BET Studies. The BET surface areas for NZP and PAM/NZP are 78 and 201 m2/g, respectively. The surface area of the doped ZnO nanoparticles was increased by the addition of polyacrylamide. This addition stabilizes textural structure,

and particle agglomeration is prevented. The pore structure is collapsed during the calcination process. 3.1.5. Thermal Stability (TGA) Analysis. The thermograms of PAM and PAM/NZP are represented in Figure 4b. It can be clearly inferred that PAM/NZP composite is thermally more stable than the polymer. The weight loss at 85 °C in the case of polyacrylamide was attributed to loss of moisture, while a weak drop in the TGA curve around 200 °C can be related to thermal processes involving melting of the PAM chains. A weight loss of 29% was observed at 350 °C. Thus, PAM showed typically four steps of weight loss.38 The weight loss at 200 °C is due to thermo-oxidative cleavage of the weak and unstable linkages, as methylene groups and side chain of amino acid residues. At 500 °C, there is a weight loss of 36.26% (±0.05%) 15552

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spherical with some agglomeration. The SEM micrograph of PAM/NZP shows the formation of a hybrid nanocomposite. The PAM exhibits rough sheetlike morphology. The NZP granules are uniformly distributed over the matrix. The EDX pattern of NZP (Figure 5c) shows the presence of Zn, Ni, and O in the sample. The stoichiometry in the final product was confirmed. The EDX pattern of PAM/NZP (Figure 5d) shows the clear presence of Zn, Ni, C, N, and O in the sample. 3.1.7. TEM Analysis. The high-resolution transmission electron micrographs of NZP and PAM/NZP are given in panels a and c of Figure 6, respectively. The particles have mixed shapes as quasi-spherical and hexagonal. The fringes and defects are clearly visible in HRTEM. The SAED pattern (Figure 6b) reveals the sample is highly crystalline. The aggregation of nanoparticles can be due to the sintering process. The TEM image of PAM/NZP (Figure 6c) shows that polyacrylamide is in a gel form and NZPs are dispersed in the polymer matrix. In the micrograph of PAM/NZP it is clearly visible that NZPs are distributed throughout the polymer matrix and the aggregation is also reduced. However, NZP nanoparticles are visible on higher resolutions because of dominance of the polymer matrix. From the SAED pattern of the composite, it has been inferred that NZP is present in crystalline form, but the crystallinity is low because of interaction with the polymer matrix. The average particle size for NZP is 20−25 nm, and for PAM/NZP the average particle size is 30−40 nm, which are in accordance with XRD results. The lattice spacing for the (101) plane has been found to be 0.235 nm from HRTEM, which matches well the XRD data (inset of Figure 6a). 3.2. Dye Removal Tests. The dye removal photoactivity of NZP and PAM/NZP was evaluated by studying the photodegradation of dyes RB and MG as target pollutants. The effect of adsorption onto the photocatalysis was also studied by exposing the photocatalytic reaction to the following conditions: equilibrium adsorption in the dark followed by photocatalysis and coupled adsorption and photocatalysis. The decrease in RB and MG concentration was measured as a fall in absorbance in the presence of the catalysts in the dark as well as sunlight. We have also tested the degradation of MG and RB in sunlight, and a 5.11% and 4.37% dye degradation, respectively, was observed. 3.2.1. Dark Adsorption Followed by Visible Light Photocatalysis (A/P). In this condition dye solutions containing the photocatalyst were kept in the dark for 1 h with constant stirring and then exposed to natural sunlight for further photodegradation. Panels a and c of Figure 7 depict percent adsorption (A) of RB and MG in the presence of NZP, respectively, showing its poor adsorption capacity. Only 6.5% of MG and 7.2% of RB was adsorbed by NZP, indicating insignificant results. Subsequent photocatalysis (P) in sunlight resulted in 71.22% of MG (Figure 7b) and 72.47% of RB (Figure 7d) degradation in 4 h of illumination. The percent degradation of dye is calculated using the following formula c − ct %degradation = 0 × 100 c0 (4)

Figure 3. XRD diffractogram of NZP, PAM, and PAM/NZP.

Figure 4. (a) UV−vis spectra and (b) TGA thermogram.

which is attributed to the oxidation of residual carbon. However, a total 19.02% (±0.05%) weight loss is reported in the case of PAM/NZP. 3.1.6. SEM/EDX Analysis. To obtain detailed information about the microstructure and morphology of the synthesized samples, scanning electron microscopy was carried out. The SEM images of ZnO and PAM/NZP are represented in panels a and b of Figure 5, respectively. NZP nanoparticles are

where C0 is initial concentration of dye before illumination and Ct is concentration of dye after time t. When NZPs were irradiated with UV−visible light, electron− hole pairs are generated, which react with water to produce 15553

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Figure 5. (a) SEM image of NZP, (b) SEM image of PAM/NZP, (c) EDX pattern of NZP, and (d) EDX pattern of PAM/NZP.

the interfacial charge transfer is in competition with the recombination of electron−hole pairs, the presence of chemical additives (e.g., transition metal ions) as electron acceptors improves the charge separation yield, causing significant enhancement of the production of ·OH. The photocatalytic reactions occurring on semiconductors are characterized by two parallel mechanisms involving reduction and oxidation. Ni2+ ions function as electron scavengers to retard the recombination of charge carriers, resulting in magnitude improvement in the photocatalytic activity of ZnO. However, because of the small amount of dopant (2% Ni), it cannot act as electron scavenger. When dye solutions containing PAM/NZP were kept in the dark, 29.56% of MG (Figure 7a) and 31.35% of RB (Figure 7c) was adsorbed in 1 h. After 4 h of further exposure to solar light, 93.67% of MG (Figure 7b) and 95.66% of RB (Figure 7d) was degraded. The excellent adsorbing properties of the crosslinked polyacrylamide gel40,41 lead to better results. The porous and cross-linked structure of polyacrylamide facilitates adsorption of pollutants through surface active sites for adsorption, ease of transportation of reactants, and harvesting of sunlight by multiple scattering of light within the porous framework. It can be inferred from BET studies that the composite has higher surface area and thus facilitates the higher adsorption of dyes onto the surface. For analyzing the photoactivity of the composite, we need to study the effect of adsorption on dye removal. The rate of photocatalytic degradation depends on concentration of dye in solution as well as on the surface of the catalyst.42−44 The total dye concentration at any time is

Figure 6. (a) HRTEM image of NZP (d-spacing with plane inset), (b) SAED pattern of NZP, (c) HRTEM image of PAM/NZP, and (d) SAED pattern of PAM/NZP.

hydroxyl and superoxide radicals which disrupt the conjugation in organic molecules such as the dye and may also mineralize them. The photocatalytic activity of ZnO has been observed less in sunlight (contains only 3−5% UV).39 The absorption was increased on doping. The effect of transition-metal ion dopants on the photocatalytic activity was the dynamics of electron−hole recombination and interfacial charge transfer. The dopant Ni ions can function as both hole and electron traps, or they can mediate interfacial charge transfer. Because

C = C b + Cs 15554

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Figure 7. (a) Adsorption (A) of MG onto NZP and PAM/NZP in the dark, (b) photodegradation (P) of MG in the presence of NZP and PAM/ NZP under sunlight, (c) adsorption of RB onto NZP and PAM/NZP in the dark, and (d) photodegradation of RB in the presence of NZP and PAM/NZP under sunlight (initial concentration of RB, 10−5 M; pH 7; temperature, 30 ± 0.5 °C).

where Cb and Cs are the amount of dye in bulk and on the surface, respectively. The amount of dye adsorbed onto the catalyst surface is calculated as45 Cs = (C0 − Ce)

V M

(6)

where V is the volume of solution and M is the mass of the adsorbent Table S1 of the Supporting Information shows the various parameters calculated for degradation of MG and RB in the presence of NZP and PAM/NZP under both reaction conditions. It has been reported that the rate of photodegradation of various dyes fitted a pseud-first-order kinetic model46 ln(C0/Ct ) = kappt

(7)

where kapp is the apparent rate constant, C0 the concentration of dyes before illumination, and Ct the concentration of dyes at time t. The overall degradation of RB and MG in the presence of NZP follows pseduo-first-order kinetics. The rate constants calculated from kinetics plots for degradation of RB (Figure 8a) and MG (Figure 8b) are 0.042 and 0.054 min−1 respectively. PAM/NZP had adsorption of MG and RB that was far higher than that of NZP (Table S1 of the Supporting Information). The difference in adsorption capacity might be due to surface charge. The surface of the composite will be negatively charged in the pH range of 6.0−7.0, causing high adsorption of cationic dyes. When the kinetics of degradation by NZP and PAM/NZP are compared, it is observed that rate constants for photodegradation of MG (Figure 9b) and RB

Figure 8. Pseudo-first-order kinetics for photodegradation (A/P) of (a) RB in the presence of NZP and PAM/NZP and (b) MG in the presence of NZP and PAM/NZP (initial concentration of RB and MG, 10−5 M; pH 7; temperature, 30 ± 0.5 °C).

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Figure 9. Degradation and kinetics plots for RB and MG under coupled process in the presence of PAM/NZP. (a) Spectrum of RB, (b) spectrum of MG, (c) percent removal of RB, (d) percent removal of MG, (e) pseudo-first-order kinetics for degradation of RB, and (f) pseudo-first-order kinetics for degradation of MG (initial concentration of RB and MG, 10−5 M; pH 7; temperature, 30 ± 0.5 °C).

clearly reveal a higher rate of photodegradation under synergistic reaction conditions. Both RB and MG are cationic dyes, and a negative charge on the surface of the photocatalyst would facilitate the adsorption. Gel electrophoresis was used to determine the polarity of PAM/NZP and NZP in pH range 6.0−7.0. PAM/NZP showed a displacement toward the positively charged electrode under the applied potential, suggesting that the surface of the nanocomposite is predominantly negatively charged. The dye degradation under coupled process can be schematically represented as

(Figure 9a) in the presence of PAM/NZP are 0.0701 and 0.0848 min−1, respectively (Table S2 of the Supporting Information). The rate constants are almost twice those for NZP. 3.2.2. Coupled Adsorption and Photocatalysis. In the second reaction condition, a synergistic effect of adsorption and photocatalysis on dye removal was studied. Panels c and d of Figure 9 show the extent of degradation of MG and RB, respectively, during coupled process in the presence of PAM/ NZP under coupled adsorption and photocatalysis (AP). A remarkable 94.55% of MG and 99.17% of RB were removed in 2 h of solar light exposure under coupled process. The overall process of dye degradation under coupled adsorption and photocatalysis follows a pseduo-first-order kinetics with overall rate constant (k0) 0.102 and 0.148 min−1 for MG (Figure 9f) and RB (Figure 9e) removal, respectively. The rate constants

PAM/NZP + D → PAM/NZP−Dads hv

PAM/NZP − Dads → PAM/NZP−D*ads 15556

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Figure 10. Proposed mechanism for degradation of dye under coupled process. − PAM/NZP − D*ads → PAM/NZP−D+· ads + eCB

mechanism has been pictorially represented in Figure 10. The negatively charged amide and hydroxyl groups become potentially active sites and could be attracted by the cationic ammonium groups in the absorbents.41 As from gel electrophoresis, no protonation of NH2 groups has occurred, so they can easily bind to cationic dyes such as RB and MG. The Ni ions present in semiconductor zinc oxide suppress the electron−hole pair recombination and bring out faster degradation of dyes. CO2 gas generation was also observed during degradation reaction of the dyes with NZP and PAM/NZP. CO2 gas was identified by saturated BaCl2 solution test via white BaCO3 precipitate formation. Our results for adsorption and degradation over NZP and PAM/NZP were in agreement with the studies by Shon et al.50 They reported simultaneous adsorption by activated carbon photodegradation by TiO2, which leads to higher degradation. The results from study of dye removal under both reaction conditions reveal that adsorption has a positive effect on overall photodegradation of dyes by PAM/NZP. The coupled process (AP) has resulted in higher and faster degradation of RB and MG. In the first reaction condition (equilibrium adsorption in the dark followed by photocatalysis) (A/P), degradation is suppressed after some time because of accumulation of dye on

e−CB + O2 → O−· 2 +· O−· 2 + Dads → degradation product

Both dyes RB and MG have tertiary amine group in their structure. At pH 6−7, the surface of the photocatalyst is negatively charged because of the presence of OH groups and amine groups, which leads to binding of these cationic dyes onto the surface. This adsorbed dye is more prone to radical attack than the suspended one. The photo excitation of the organic dye plays an important role in degradation of the dye under the AP system. The electron transfer leads to formation of adsorbed dye radical cation, which further produces oxygen radical anion which brings about degradation of organic dye by disruption of conjugation. Adsorption of pollutant onto the catalyst surface can enhance the photodegradation.45,46 In an alternate explanation, it can be suggested that as soon as the dye is adsorbed onto PAM/NZP and the system is exposed to sunlight, there is a simultaneous production of an electron−hole pair in NZP. This brings about the degradation of adsorbed dye faster than that for the dye present in the bulk solution. There is a generation of OH* free radicals leading to disruption of conjugation in organic pollutants.47−49 This 15557

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Table 1. Different Photocatalytic Systems for Dye Removal photocatalyst

irradiation source

dye

decolorisation time (min)

dye conc

amount of catalyst (mg/L)

degusa P-25 Ag (2%) doped ZnO Cu(0.5%) doped ZnO Fe(0.7%) doped ZnO PANI/ZnO

UV UV UV UV solar light solar light

12 mg/L 30 mg/L 20 mg/L 20 ppm 10−5 M 10−5 M 10−5 M

2 g/L 1 g/L 1 g/L − 0.4 mg/mL 0.4 mg/mL 1 g/L

Ag/TiO2/β-CD Fe3O4-MWCNT PAM/NZP

solar light UV solar light

90 60 240 120 240 240 360 360 225 120 120

55 57 54 53 56

PANI/TiO2

methylene blue methylene blue methyl orange methylene orange methylene blue malachite green methylene blue rhodamine B methylene blue rhodamine B malachite green rhodamine B

ref

10 ppm 10 mg/L 10−5 M

0.2 g/L 2.5 g/L 0.25 g/L

51 52 present study

14

4. CONCLUSION The coupled or synergistic effect of adsorption and photocatalysis has produced promising results for wastewater treatment. The novel PAM/NZP nanocomposite has an efficient adsorbing power and visible light photoactivity. The utilization of photon energy from a wider spectrum of solar light, lower electron−hole recombination, and simultaneous adsorption leads to significant results in degradation of rhodamine B and malachite green. NZP nanoparticles also exhibited a decent photodegradation activity but scores low because of poor adsorption. In fact, the PAM/NZP has longterm reusability and recovery. The simple and economic methodology of synthesis and the synergistic effect of adsorption and photodegradation action make this nanocomposite an excellent material for wastewater treatment.

the surface of the photocatalyst. However, in the case of the coupled process, the instantaneous adsorbed amount of dye onto the photocatalyst is so small that chances of deactivation of catalyst due to accumulation are decreased. Thus, PAM/ NZP produces excellent results under the synergistic or coupled adsorption and photocatalysis. When the results51−57 are compared with various other reported systems, it can be inferred that the present study has better results under solar light irradiation. The results of earlier works are reported in Table 1. 3.3. Reusability of Photocataysts. To evaluate the photodegradation resuablilty and long-term usage of PAM/ NZP, it was reused for several photocatalytic experiments for illumination of 8 h. Figure 11 shows the recycling efficiency of

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ASSOCIATED CONTENT

S Supporting Information *

Various parameters calculated for degradation of MG and RB in the presence of NZP and PAM/NZP under both reaction conditions (Table S1), apparent rate constants and linear regression coefficients from log C0/Ct versus T plots for different dye concentrations (Table S2), structure of the dyes (Figure S1 and S2), and digital photographs of RB and MG solutions (Figures S3 and S4). This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

Figure 11. RB and MG degradation for recycling experiments of NZP and PAM/NZP (initial concentration of RB and MG, 10−5 M; pH 7; temperature, 30 ± 0.5 °C).

*E-mail: [email protected]. Phone: +919625310313. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank Shoolini University for providing necessary facilities and support. The authors also acknowledge National Institute of Pharmaceutical and Education Research, Mohali, India for providing facilities. One of the authors (Mu. Naushad) acknowledges the King Saud University, Deanship of Scientific Research, College of Science Research Center for the financial support.

the PAM/NZP and NZP for degradation of RB and MG. After 8 h of irradiation, almost complete dye removal for the first usage is achieved; for the sixth usage, 92.16% of RB and 88.76% of MG is removed. The retardation of photocatalytic activity on subsequent usages is due to accumulation of intermediates and dye molecules on the surface of the photocatalyst leading to a decrease in concentration of OH*.58,59 However, a loss of over 20% is observed for NZP. Hence, reusability and recovery of PAM/NZP is easier. PAM is a popular flocculant. The high adsorption capacity and solar light photoactivity of PAM/NZP makes it thus more suitable for pollutant removal from aqueous medium.

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